Etch of silicon nitride selective to silicon and silicon dioxide useful during the formation of a semiconductor device

ABSTRACT

A method for etching silicon nitride selective to silicon dioxide and silicon (polycrystalline silicon or monocrystalline silicon) comprises the use of oxygen along with an additional etchant of either CHF 3  or CH 2 F 2 . Flow rates, power, and pressure settings are specified.

FIELD OF THE INVENTION

[0001] This invention relates to the field of semiconductor manufactureand, more particularly, to an etch useful for removing silicon nitrideselective to silicon and silicon dioxide.

BACKGROUND OF THE INVENTION

[0002] During the manufacture of semiconductor devices such as a memorydevices, logic devices, and microprocessors, various processes arecommonly performed. Etching silicon nitride selective to silicon (suchas polysilicon) and to silicon dioxide with various etch ratios is oftenrequired. For example, hot phosphoric acid isotropically etches siliconnitride selective to silicon dioxide and silicon. Other processes foretching Si₃N₄ selective to SiO₂ and Si are discussed in “HighlySelective Etching of Silicon Nitride Over Silicon and Silicon Dioxide,”J. Vac. Sci. Technol. A 17(6), Nov/Dec 1999, which describes the use ofoxygen (O₂) and nitrogen (N₂) in combination with CF₄ or NF₃. Theprocesses discussed achieve Si₃N₄ to Si and to SiO₂ etch rate ratios ofup to 100 and 70 respectively using nitrogen trifluoride. The processesuses high flow rates of 800 standard cubic centimeters (sccm) O₂ and 110sccm N₂ for most experiments. Further, using carbon tetrafluoride, aSi₃N₄ to polysilicon etch ratio of 40 was achieved while SiO₂ was notetched at all. Etch rates of silicon nitride are below about 50angstroms (Å) per minute.

[0003] As semiconductor manufacture typically requires high volumeprocessing to lower costs, any decrease in temporal processingrequirements can result in a large increase in product throughput.Further, having options available to accomplish a task such as etchingsilicon nitride is advantageous, as one process may function better forcertain manufacturing flows. Additional methods for etching siliconnitride selective to silicon and silicon nitride at an accelerated ratewould be desirable.

SUMMARY OF THE INVENTION

[0004] The present invention provides a new etch method that, amongother advantages, reduces problems associated with the manufacture ofsemiconductor devices, particularly problems in etching silicon nitrideselective to silicon and silicon dioxide. In accordance with oneembodiment of the invention a semiconductor wafer substrate assemblyhaving a layer of silicon nitride and a layer of at least one of siliconand silicon dioxide is placed into an etch chamber. Oxygen and eitherCHF₃ or CH₂F₂ are introduced into an etch chamber under controlled flowrates, power, and pressure. At the parameters detailed herein, the etchremoves silicon nitride selective to silicon dioxide and silicon.

[0005] Additional advantages will become apparent to those skilled inthe art from the following detailed description read in conjunction withthe appended claims and the drawings attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 depicts a cross section of a semiconductor wafer assemblyincluding a blanket layer of silicon nitride; and

[0007]FIG. 2 depicts the cross section of FIG. 1 subsequent to an etchto form silicon nitride spacers.

[0008] It should be emphasized that the drawings herein may not be toexact scale and are schematic representations. The drawings are notintended to portray the specific parameters, materials, particular uses,or the structural details of the invention, which can be determined byone of skill in the art by examination of the information herein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0009] A silicon nitride etch selective to silicon (such asmonocrystalline or polycrystalline silicon) and to silicon dioxidecomprises the use of O₂ and either CHF₃ or CH₂F₂ at relatively low flowrates. It should be noted that the parameters described herein areoptimized for an Applied Materials 5000 (AME5000) etcher, but they maybe converted easily by one of ordinary skill in the art for othersystems.

[0010] In accordance with one embodiment of the invention, asemiconductor wafer having a layer of silicon nitride and a layer ofsilicon dioxide and/or silicon is placed into an etch chamber andsubjected to an etch. This embodiment of the etch comprises an O₂:CHF₃or O₂:CH₂F₂ flow rate ratio of greater than about 3:1, which results ina relatively rapid, controllable Si₃N₄ etch rate with good selectivityto Si and to SiO₂. For example, an O₂ flow rate of between about 20 sccmand about 80 sccm and a CHF₃ or CH₂F₂ flow of between about 5 sccm andabout 25 sccm would be sufficient. More preferably, an O₂ flow rate ofbetween about 35 sccm and about 60 sccm and a CHF₃ or CH₂F₂ flow ofbetween about 10 sccm and about 20 sccm would be sufficient, and mostpreferably an O₂ flow rate of about 60 sccm and a CHF₃ or CH₂F₂ flow ofabout 20 sccm would be sufficient.

[0011] During the etch, a pressure of between about 10 millitorr toabout 60 millitorr is maintained. More preferably, a pressure of betweenabout 30 to about 60 millitorr, and most preferably, between about 30 toabout 40 millitorr is maintained. Further, a power of between about 300watts to about 600 watts, more preferably between about 300 watts toabout 500 watts, and most preferably between about 300 to about 400watts sustained within the chamber. With increasing power and/orpressure the etch rate increases and the selectivity to Si and SiO₂decreases.

[0012] Using the parameters described above, it is estimated that anSi₃N₄:SiO₂ etch ratio of up to about 1.3:1 on a blanket film and up toabout 3:1 over topography can be achieved, as well as an Si₃N₄:Si etchratio of up to about 7:1. As the O₂:CHF₃ or O₂:CH₂F₂ increases beyond3:1 the etch rate of the Si₃N₄ decreases and the selectivity to Si andSiO₂ increases. With a high O₂:CHF₃ or O₂:CH₂F₂ ratio, above about 10:1,the etch rate becomes so slow as to be decreasing usefulness in mostinstances. As the O₂:CHF₃ or O₂:CH₂F₂ ratio decreases further below 3:1the etch removes Si₃N₄ at an increased rate but also becomes lessselective to Si and SiO₂. An O₂:CHF₃ or O₂:CH₂F₂ ratio of about 3:1 isbelieved to provide a good balance between nitride etch rate andselectivity to Si and SiO₂ in many uses of the invention forsemiconductor manufacture.

[0013] At a pressure of about 30 millitorr, a power of 300 watts, a CHF₃or CH₂F₂ flow rate of 20 sccm, and an O₂ flow rate of 60, the etch rateof Si₃N₄ will be about 420 Å/min. With a pressure of 40 millitorr, apressure of about 40 millitorr, a power of 300 watts, a CHF₃ or CH₂F₂flow rate of about 10 sccm and an O₂ flow rate of about 40 sccm, theetch rate increases to about 904 Å/min. As the CHF₃/O₂ or CH₂F₂/O₂ ratioincreases the Si₃N₄ etch rate increases, such that at a pressure of 30millitorr and a power of 300 watts, and a flow rate of 35 sccm for bothO₂ and CHF₃ or CH₂F₂ the etch rate increases to about 1270 Å/min. Theseprocesses use a chuck temperature of about 10° C. and a sidewalltemperature of about 20° C.

[0014] Using the preferred parameters of the instant invention asdiscussed above results in an anisotropic nitride etch which removesnitride from horizontal surfaces faster than it removes nitride fromvertical surfaces at an etch ratio of about 3.7:1. This is especiallyuseful for etching structures similar to those depicted in FIGS. 1 and2, as it allows sufficient etching to remove the nitride layer from thewafer surface without damaging the silicon wafer. The structure of FIG.1 depicts a semiconductor substrate assembly 10 comprising asemiconductor wafer 12, field oxide 14, doped transistor source/drainregions 16 within the wafer 12, and a transistor gate stack comprisinggate oxide 18, polycrystalline silicon (polysilicon) 20, a conductivesilicide layer 22, and an oxide or nitride layer 24, for exampletetraethyl orthosilicate (TEOS) or silicon nitride. FIG. 1 furtherdepicts a blanket silicon nitride layer between about 300 Å about 600 Åthick. Such a structure can be manufactured by one of ordinary skill inthe art.

[0015] The structure of FIG. 1 is subjected to an inventive etch asdescribed above. An exemplary etch includes processing the wafer in achamber of an AME5000 etch chamber. After placing the wafer substrateassembly in the etch chamber, O₂ and CHF₃ or CH₂F₂ are introduced intothe chamber at flow rates of about 60 sccm and about 20 sccmrespectively. Pressure is maintained at between about 30 millitorr andabout 40 millitorr, and a power of between about 300 watts and about 400watts is utilized. At a chuck temperature of about 10° C. and a sidewalltemperature of about 20° C., the silicon nitride will etch at a rate ofabout 720 Å/min in the vertical direction, and about 180 Å/min in thehorizontal direction. Generally, the vertical:horizontal etch rate willbe about 4:1. For the 525 Å thick layer of silicon nitride depicted inFIG. 1, the etch is performed for between about 35 seconds and about 60seconds which results in the structure of FIG. 2. Spacers 32 having awidth of about 300 Å to about 400 Å are formed.

[0016] The etch detailed above provides a silicon nitride etch which isselective to silicon and silicon dioxide. This is accomplished using theetch as described consisting essentially of a flow of O₂ and CHF₃ orCH₂F₂. The addition of hydrogen from CHF₃ or from CH₂F₂, in addition toproviding an etch of silicon nitride selective to silicon and silicondioxide, further provides an etch which results in a more square profileof the completed transistor structures than conventional etches. Therelatively low flow rates of CHF₃ or CH₂F₂ may also contribute to theless rounded feature profile compared with conventional etches havinghigh gas flow rates. Rounded profiles may adversely affect theself-aligned contact (SAC) etch performance, and possibly affect theelectrical properties of the device. In extreme cases, conventionaletches can expose the conductive polysilicon, metal, or silicide whichis protected by the oxide. Additionally, the structure depicted in FIG.1, and other similar structures, can be etched with less concern fordamaging the silicon wafer once the nitride has been cleared due to thehigh selectivity to silicon.

[0017] A semiconductor device formed in accordance with the inventionmay be attached along with other devices to a printed circuit board, forexample to a computer motherboard or as a part of a memory module usedin a personal computer, a minicomputer, or a mainframe. A device formedin accordance with the invention could further be useful in otherelectronic devices related to telecommunications, the automobileindustry, semiconductor test and manufacturing equipment, consumerelectronics, and virtually any consumer or industrial electronicequipment.

[0018] While this invention has been described with reference toillustrative embodiments, this description is not meant to be construedin a limiting sense. Various modifications of the illustrativeembodiments, as well as additional embodiments of the invention, will beapparent to persons skilled in the art upon reference to thisdescription. It is therefore contemplated that the appended claims willcover any such modifications or embodiments as fall within the truescope of the invention.

What is claimed is:
 1. A method for etching a layer of silicon nitridecomprising: etching said silicon nitride layer with an etchantconsisting essentially of oxygen at a flow rate of between about 20 sccmand about 80 sccm and one of CHF₃ and CH₂F₂ at a flow rate of betweenabout 5 sccm and about 25 sccm.
 2. The method of claim 1 furthercomprising introducing said oxygen and said one of CHF₃ and CH₂F₂ intoan etch chamber at a ratio of about 3:1 during said etching.
 3. Themethod of claim 1 further comprising subjecting said silicon nitridelayer to a pressure of between about 10 millitorr and 60 millitorr and apower of between about 300 watts and about 600 watts during saidetching.
 4. The method of claim 1 further comprising introducing saidoxygen into an etch chamber at a flow rate of between about 35 sccm andabout 60 sccm and introducing said one of CHF₃ and CH₂F₂ into said etchchamber at a flow rate of between about 10 and about 20 sccm during saidetching.
 5. The method of claim 4 further comprising subjecting saidsilicon nitride layer to a pressure of between about 30 millitorr and 60millitorr and a power of between about 300 watts and about 500 wattsduring said etching.
 6. The method of claim 1 further comprisingintroducing oxygen into an etch chamber at a flow rate of about 60 sccmand introducing said one of CHF₃ and CH₂F₂ into said etch chamber at aflow rate of about 20 sccm during said etching.
 7. The method of claim 6further comprising subjecting said silicon nitride layer to a pressureof between about 30 and 40 millitorr and a power of between about 300watts and about 400 watts during said etching.
 8. A method used duringthe formation of a semiconductor device comprising: providing asemiconductor wafer assembly comprising at least one of a layer ofsilicon and a layer of silicon dioxide; forming a layer of siliconnitride over said at least one of said layer of silicon and said layerof silicon dioxide; etching said silicon nitride with an etch consistingessentially of oxygen and one of CHF₃ and CH₂F₂, wherein said etchexposes said at least one of said layer of silicon and said layer ofsilicon dioxide.
 9. A method used during the formation of asemiconductor device comprising: providing a semiconductor waferassembly comprising a silicon wafer and a layer of silicon dioxideoverlying said wafer; forming a layer of silicon nitride over saidsilicon wafer and over said layer of silicon dioxide; placing saidsemiconductor wafer assembly into an etch chamber; etching said siliconnitride layer using an etch consisting essentially of oxygen and one ofCHF₃ and CH₂F₂ to expose said silicon dioxide layer and said siliconwafer.
 10. The method of claim 9 further comprising introducing saidoxygen and said one of CHF₃ and CH₂F₂ into said etch chamber at a ratioof about 3:1 during said etching.
 11. The method of claim 9 furthercomprising introducing oxygen into said chamber at a flow rate ofbetween about 20 sccm and about 80 sccm and introducing said one of CHF₃and CH₂F₂ into said etch chamber at a flow rate of between about 5 sccmand about 25 sccm during said etching.
 12. The method of claim 11further comprising subjecting said silicon nitride layer to a pressureof between about 10 millitorr and 60 millitorr and a power of betweenabout 300 watts and about 600 watts during said etching.
 13. The methodof claim 9 further comprising introducing said oxygen into said chamberat a flow rate of between about 35 sccm and about 60 sccm andintroducing said one of CHF₃ and CH₂F₂ into said etch chamber at a flowrate of between about 10 sccm and about 20 sccm during said etching. 14.The method of claim 13 further comprising subjecting said siliconnitride layer to a pressure of between about 30 millitorr and 60millitorr and a power of between about 300 watts and about 500 wattsduring said etching.
 15. The method of claim 9 further comprisingintroducing said oxygen into said chamber at a flow rate of about 60sccm and introducing said one of CHF₃ and CH₂F₂ into said etch chamberat a flow rate of about 20 sccm during said etching.
 16. The method ofclaim 15 further comprising subjecting said silicon nitride layer to apressure of between about 30 millitorr and 40 millitorr and a power ofbetween about 300 watts and about 400 watts during said etching.